1. Field of the Invention
The subject of the present invention is a cryogenic tunnel for chilling products, especially food products.
2. Description of the Related Art
Tunnels of this type generally comprise a horizontal conveyor belt for the products and means for blowing a cold gas thereover and for limiting the escape of cold gas from the tunnel and/or the admission of external gas into the tunnel.
This is because it is necessary to prevent the natural escape of cold gas from the tunnel and therefore to direct the cold air back into the tunnel, in order to recover heat transfer energy.
The purpose of this recirculation of cold air is therefore to improve the heat transfer coefficient, but without having to add excessive mechanical energy to the gas flow system. This is because it is important that the lightweight products moving on the conveyor belt cannot be deflected from their paths by employing these means to redirect the cold gas. Finally, it is also necessary to prevent the formation of ice in the tunnel, which could be caused by the ingress of moist external air mixing in the tunnel with the escaping cold air. Such ice formation is undesirable since it increases the energy consumption needed.
It is therefore important to prevent the entry of moist air from outside the tunnel.
To solve this problem various solutions have hitherto been proposed, especially the following: inclining the tunnel in order to help the cold, heavy air to move under gravity in a controlled direction, the use of gas deflector plates or louvers opposite fans, the use of centrifugal blowers and pipes to suck up the gas and blow it in the desired direction, these devices being made bidirectional by employing mechanical baffles. The systems which have turned out to be the most satisfactory are centrifugal blowers, but they have the drawback of tending to move the lightweight products on the conveyor belt. Furthermore, they produce localised variations in flow and are difficult to clean.
The object of the invention is therefore to fit the cryogenic tunnel with means for controlling the flow of cold gas which do not have these drawbacks.
In accordance with the invention, the cryogenic tunnel comprises at least one fan placed above the conveyor belt, the blades of which extend transversely with respect to the belt over virtually the entire width thereof.
According to one preferred embodiment, the fan has two to six radial blades whose axis of rotation is virtually perpendicular to the longitudinal direction of progression of the belt and parallel thereto.
Thus the fan is fitted with a small number of blades, but they extend over practically the entire width of the belt, and rotate at a suitable speed so that excess energy is not dissipated over the moving products. Cold gas is thus gently moved over the entire surface of the belt, without any strong sucking or strong blowing region which could cause undesirable swirls of the hot or cold gas, or cause movements of the products on the belt. This fan creates an increased heat transfer region by the action of its vanes and effectively opposes the escape of cold gas from the tunnel.
The tunnel may comprise one or more of the following characteristics:
the fan has radial blades whose axis of rotation is virtually perpendicular to the longitudinal direction of progression of the belt and parallel thereto;
the fan has two to six blades;
the tunnel comprises means of reversing the direction of rotation of the fan;
the tunnel comprises means of varying the speed of rotation of the fan;
the tunnel comprises at least two fans as defined, placed in longitudinally spaced-out positions in the tunnel;
each fan is placed close to one end of the tunnel.